The present invention relates generally to aqueous electrolytes in electrochemical systems, and particularly to a method and apparatus for analyzing the zinc concentration of an aqueous zinc-halogen electrolyte in zinc-halogen secondary energy storage battery systems in order to determine the state of charge for these battery systems.
The secondary energy storage systems of the type referred to herein (e.g., a zinc-chloride battery system or other zinc-halogen battery system) generally are comprised of three basic components, namely an electrode stack section, an electrolyte circulation subsystem, and a store subsystem. The electrode stack section typically includes a plurality of cells connected together electrically in various series and parallel combinations to achieve a desired operating voltage and current at the battery terminals over a charge/discharge battery cycle. Each cell is comprised of a positive and negative electrode which are both in contact with an aqueous zinc-halogen electrolyte. The electrolyte circulation subsystem operates to circulate the zinc-halogen electrolyte from a reservoir through each of the cells in the electrode stack in order to replenish the zinc and halogen electrolyte components as they are oxidized or reduced in the cells during the battery cycle. In a closed, self-contained zinc-halogen battery system, the storage subsystem is used to contain the halogen gas or liquid which is liberated from the cells during the charging of the battery system for subsequent return to the cells during the discharging of the battery system. In the zinc-chloride battery system, chlorine gas is liberated from the positive electrodes of the cells and stored in the form of chlorine hydrate. Chlorine hydrate is a solid which is formed by the store subsystem in a process analogous to the process of freezing water where chlorine is included in the ice crystal.
With reference to the general operation of a zinc-chloride battery system, an electrolyte pump operates to circulate the aqueous zinc-chloride electrolyte from a reservoir to each of the positive "chlorine" electrodes in the electrode stack. These chlorine electrodes are typically made of porous graphite, and the electrolyte passes through the pores of the chlorine electrodes into a space between the chlorine electrodes and the opposing negative or "zinc" electrodes. The electrolyte then flows up between the opposing electrodes or otherwise out of the cells in the electrode stack and back to the electrolyte reservoir or sump.
During the charging of the zinc-chloride battery system, zinc metal is deposited on the zinc electrode substrates and chlorine gas is liberated or generated at the chlorine electrode. The chlorine gas is collected in a suitable conduit, and then mixed with a chilled liquid to form chlorine hydrate. A gas pump is typically employed to draw the chlorine gas from the electrode stack and mix it with the chilled liquid, (i.e., generally either zinc-chloride electrolyte or water). The chlorine hydrate is then deposited in a store container until the battery system is to be discharged.
During the discharging of the zinc-chloride battery system, the chlorine hydrate is decomposed by permitting temperature to increase, such as by circulating a warm liquid through the store container. The chlorine gas thereby recovered is returned to the electrode stack via the electrolyte circulation subsystem, where it is reduced at the chlorine electrodes. Simultaneously, the zinc metal is dissolved off of the zinc electrode substrates, and power is available at the battery terminals.
Further discussion of the structure and operation of zinc-chloride battery systems may be found in the following commonly assigned patents: Symons U.S. Pat. No. 3,713,888 entitled "Process For Electrical Energy Using Solid Halogen Hydrates"; Symons U.S. Pat. No. 3,809,578 entitled "Process For Forming And Storing Halogen Hydrate In A Battery"; Carr et al U.S. Pat. No. 3,881,958 entitled "Mass Flow Rate Control Of Chlorine Content Of Electrolyte For High Energy Density Battery"; Carr U.S. Pat. No. 4,100,332 entitled "Comb Type Bipolar Electrode Elements And Battery Stack Thereof". Such systems are also described in published reports prepared by the assignee herein, such as "Development of the Zinc-Chloride Battery for Utility Applications," Interim Report EM-1417, May 1980, and "Development of the Zinc-Chloride Battery for Utility Applications," Interim Report EM-1051, April 1979, both prepared for the Electric Power Research Institute, Palo Alto, Calif. The specific teachings of the aforementioned cited references are incorporated herein by reference.
Over the course of the zinc-chloride battery charge/discharge cycle, the concentration of the electrolyte varies as a result of the electrochemical reactions occurring at the electrodes in the cells of the electrode stack. At the beginning of charge, the concentration of zinc-chloride in the aqueous electrolyte may typically be 2.0 Molar. As the charging portion of the cycle progresses, the electrolyte concentration will gradually decrease with the depletion of zinc and chloride ions from the electrolyte. When the battery system is fully charged, the electrolyte concentration will typically be reduced to 0.5 Molar. Then, as the battery system is discharged, the electrolyte concentration will gradually swing upwardly and return to the original 2.0 Molar concentration when the battery system is completely or fully discharged.
The present invention is directed to a method and apparatus for analyzing or determining the zinc concentration in the electrolyte of zinc chloride and other zinc-halogen battery systems to determine the state of charge of these battery systems. While in many other battery systems resort must be had to detecting sensitive changes in the overall voltage of the battery to determine the state of charge, it will be appreciated that the variation in the electrolyte concentration described above for the zinc-chloride battery system provides for an accurate and reliable means for determining the state of charge. In the laboratory, the electrolyte concentration may be readily determined by employing a hydrometer and calibrating the specific gravity to the Molar concentration of the electrolyte. However, this technique is not practical for a commercial battery system.
Accordingly, it is a principle object of the present invention to provide an apparatus and method of analyzing the zinc concentration in an aqueous zinc-halogen electrolyte which may be employed in closed self-contained secondary energy storage battery system in order to determine the state of charge for the battery system.
It is a more specific object of the present invention to provide an electrochemical apparatus and method for analyzing the zinc concentration in an aqueous zinc-chloride electrolyte which is accurate and reliable even when the electrolyte to be analyzed is saturated with dissolved chlorine.
It is another object of the present invention to provide an electrochemical apparatus and method of analyzing the zinc concentration in an aqueous zinc-chloride electrolyte where a flow of the electrolyte to be analyzed is provided through the apparatus.
It is a further object of the present invention to provide an electrochemical apparatus and method of analyzing the zinc concentration in an aqueous zinc-chloride electrolyte which is operable from a low voltage d.c. power source, such as a conventional portable battery.
It is an additional object of the present invention to provide an electrochemical apparatus and method of analyzing the zinc concentration in an aqueous zinc-chloride electrolyte of a zinc-chloride battery system which will not contaminate or otherwise introduce foreign matter into the battery system.
To achieve the foregoing objects, the present invention provides an electrochemical apparatus for analyzing or determining the zinc concentration in an aqueous zinc-halogen electrolyte which includes a working electrode and an auxiliary electrode both in contact with the electrolyte to be analyzed, a reference electrode in contact with an aqueous zinc-halogen electrolyte of a predetermined concentration, and means for permitting the electrolyte to be analyzed to contact the electrolyte of predetermined concentration. The apparatus also includes circuit means for applying electrical power to the working and auxiliary electrodes for a first predetermined time period sufficient to deposit zinc metal onto the working electrode, and for measuring a potential difference across the reference and working electrodes indicative of the concentration of the electrolyte to be analyzed during a second predetermined time period. A suitable chamber means may also be provided for containing at least the portion of the electrolyte to be analyzed, with the working and auxiliary electrodes mounted in the chamber means such that they are in contact with the electrolyte within the chamber means. Similarly, vessel means may be provided for containing a supply of the electrolyte of a predetermined concentration, with the reference electrode and the vessel means associated such that the reference electrode is in contact with the electrolyte of a predetermined concentration. The vessel means is formed with liquid junction means for permitting the electrolyte of a predetermined concentration to contact the electrolyte to be analyzed within the chamber means. The predetermined concentration of the electrolyte within the vessel means may be any suitable concentration which provides for stable and consistent results. The analytical method according to the present invention includes the steps of applying electrical power to the working and auxiliary electrodes for a first predetermined time period sufficient to deposit zinc metal onto the working electrode, and measuring the potential difference across the reference and working electrodes during a second predetermined time period.
Additional advantages and features of the present invention will become apparent from a reading of the detailed description of the preferred embodiment which makes reference to the following set of drawings in which :